Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, metal sputtering, plasma oxide deposition, and plasma etching that have been developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable mirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs are often operated in a digital bistable mode of operation and as such are the core of true digital full-color image projection systems.
Many different kinds of micromirror devices exist, including torsion beam devices, and hidden-hinge devices. All micromirror devices, however, are usually operated in one of two modes of operation. The first mode of operation is an analog mode, sometimes called beam steering, wherein the address electrode is charged to a voltage corresponding to the desired deflection of the mirror. Light striking the micromirror device is reflected by the mirror at an angle determined by the deflection of the mirror. Depending on the voltage applied to the address electrode, the cone of light reflected by an individual mirror is directed to fall outside the aperture of a projection lens, partially within the aperture, or completely within the aperture of the lens. The reflected light is focused by the lens onto an image plane, with each individual mirror corresponding to a pixel on the image plane. As the cone of reflected light is moved from completely within the aperture to completely outside the aperture, the image location corresponding to the mirror dims, creating continuous brightness levels.
The second mode of operation is a digital mode. When operated digitally, each micromirror is fully deflected in either of the two directions about the torsion hinge axis. Digital operation uses a relatively large address voltage to ensure the mirror is fully deflected. The address electrodes are driven using standard logic voltage levels and a bias voltage, typically a positive voltage, is applied to the mirror metal layer to control the voltage difference between the address electrodes and the mirrors. Use of a sufficiently large mirror bias voltage, a voltage above what is termed the threshold voltage of the device, ensures the mirror will fully deflect toward the address electrode—even in the absence of an address voltage. The use of a large mirror bias voltage enables the use of low address voltages since the address voltages need only slightly deflect the mirror prior to the application of the large mirror bias voltage.
To create an image using the micromirror device, the light source is positioned at an angle relative to the device normal that is twice the angle of rotation so that mirrors rotated toward the light source reflect light in a direction normal to the surface of the micromirror device and into the aperture of a projection lens—creating a bright pixel on the image plane. Mirrors rotated away from the light source reflect light away from the projection lens—leaving the corresponding pixel dark. Intermediate brightness levels are created by pulse width modulation techniques in which the mirror rapidly is rotated on and off to vary the quantity of light reaching the image plane. The human eye integrates the light pulses and the brain perceives a flicker-free intermediate brightness level.
As can be appreciated, the temporal response of micromirrors in the micromirror array to input signals is an important metric of the performance of the micromirrors. Indeed, the temporal response characteristics can be used to determine if the micromirrors are operating properly or improperly. More specifically, the temporal response characteristics can detect manufacturing defects in the micromirrors at a macroscopic scale. The characteristics can also measure imperfections and performance variations across the surface of a micromirror array.
Existing systems for measuring the temporal response of micromirror arrays utilize devices that measure the performance of individual micromirrors in the micromirror array. For example, one method for measuring the temporal response of a micromirror uses a micromirror device characterization unit (MMDCU) or Nanospec measuring tool. These measurement tools have significant drawbacks though. In particular, the tools are expensive, highly sensitive, and require precise alignment with the micromirror array in order to produce accurate measurements. Furthermore, these tools can only measure one mirror at a time, thus producing measurement that have a relatively low signal to noise ratio.
Accordingly, there is a need in the art for a relatively simple and inexpensive method and apparatus for measuring the temporal response characteristics of a micromirror array. There is also a need in the art for a measuring device and method that can simultaneously measure the temporal response of a plurality of micromirrors thereby increasing the signal to noise ratio, and rendering a statistical ensemble-averaged response. There is also a need for a measuring device and method that can measure the temporal response of micromirrors without using sensitive microscopic and micro-positioning equipment.